Chemical structures, reaction diagrams, and photokinetics. (a) Structures of L-methionine (Met), L-azidohomoalanine (Aha), and photocaged Aha (NPPOC-Aha). (b) SPAAC is a bioorthogonal reaction for labeling azides incorporated into proteins with strained cyclooctynes. (c) Upon light exposure, photolysis of NPPOC-Aha yields free Aha for incorporation into newly synthesized proteins. (d) Kinetic analysis of NPPOC-Aha photolysis demonstrates rapid uncaging suitable for biological sampling. (e) A carboxyfluorescein-labeled bicyclononyne (FAM-BCN) provides a fluorescent tag for labeling azide-functionalized proteins.